Bridge control for electric blankets



Oct. 21, 1947.

G. c. C(ROWLEY 2,429,453

BRIDGE CONTROL FOR ELECTRIC BLANKETS Filed March 21, 1946 I A52 25 25 I8 5 4 I4 15 g 9b 9 27 Inventor: George C. CPoMey, by W His Attorny.

Patented Oct. 21, 1947 BRIDGE O T O FO ELE TRI BLANKETS George C. Crowley, Bridgeport, Conn, assignorto General Electric. Company, a corporation of New York App i t M rch. 21, 46, Seria No- 655,9 6

This invention relates to electric blankets, and I More specifically, this invention relates to elec- I tric blanket controls of the type employing an electric valve for controlling the supply of energy to the blanket, such as described and claimed in the copending application of Harry C. Anderson and Kenneth T. Sutton, Serial No. 639,992, filed January 9, 1946, and my copending application, Serial No. 639,994, filed January 9, 1946, and both of which applications are assigned to the assignee of the instant application. In those applications the firing of the electric valve, which controls the energization of the blanket, is controlled by means of a Wheatstone bridge, one leg of which is a resistance located in the blanket so as to respond to its temperature. This leg is known as a feeler circuit or element, and is not a part of the blanket heating means itself, but is a separate element.

This invention also uses a Wheatstone bridge, but the heating conductor itself constitutes one impedance element of the bridge. I prefer to divide the heater unit into two parts, and use the two parts as two of the impedance legs of the bridge. The two other impedance legs of the bridge are external of the blanket body. The two resistances of the heater legs respond differentially to changes in the blanket temperature, and this characteristic is employed to control the bridge and thereby the voltage which controls the firing of the control electric valve. When the blanket temperature rises above a predetermined high value, the valve functions, in effect, to deenergize the blanket heater, and when the temperature falls below the desired value the valve reenergizes the blanket heater- Inasmuch as the valve when it, in effect, deenergizes the blanket heater completely unbalances the Wheatstone bridge and causes it to lose its valve controlling function, I provide auxiliary means for firing the tube periodically until the blanket has cooled down sufficiently to cause the blanket heaters to reestablish the bridge and reclaim control of the contactor.

For a more complete understanding of this invention, reference should be had to the accompanying drawing in which Fig. 1 illustrates an electrically heated blanket embodying this invention; and Fig, 2 is a. diagrammatic representation of'the blanket together with the control means therefor arranged in accordance with this invention.

5 Claims. (Cl. 219.46),

Referring to the drawing, this invention has been shown in one iorm as applied to an elec-' trically heated blanket provided with a flexible blanket body I to which is applied heating means 2 consisting of a pair of resistance heatin elements 3 and 4 energized from a suitable source of alternating current, the opposite sides of which are denoted by the numerals 5 and 6. Each of the resistance conductors 3 and 4 is arranged into a number of convolutions covering the heated area of the blanket, as shown in Fig. 2. It will be understood that the blanket will be provided with suitable channels ("not shown) in which the resistance conductors 3 and 4 are threaded.

Preferably, a blanket body will be used such as described and claimed in the United States pat ent to I. O, Moberg, No. 2,203,918, dated June 11, 1940.

The heaters 3 and 4 have equal ohmic resistance values, but heater 3 is formed of a material which has a positive temperature coeificient of resistance which is relatively high as compared with that ofthe material from which heater 4 is made, which also may be positive, but which can be substantially neutral; for example, heater 3 may be made of copper, while heater 4 may be made of an alloy of copper and cadmium.

In order to control the, energization of I the heating resistances 3 and 4 so as to preventoverheating in the blanket, and so asto cy le the heating means between energized and deenergized conditions to hold a substantially uniform temperature, a Wheatstone bridge 1 is provided. Two of the impedance legs of this bridge are defined by the heating resistances 3 and 4; and the bridge is completed by two other impedance legs 8 and 9 external of the blanket, the leg 8 comprising a fixed resistance, as shown, and the. leg 9 comprising a fixed resistance 9a and an adjustable resistance 91). The bridge terminal 10 is connected with upper supply conductor 5 by means of conductor H, while bridge terminal [2 is connected with the lower supply conductor 6 by means of conductor l3. Also, terminal") is connected with one side of heater 4, the other side of which is connected with bridge terminal l4; and bridge terminal I2 is connected with one side of heater 3, the other side of which is connected with bridge terminal Ma. The two terminals 'l4 and Ila are in efiect a common terminal (I4, Ma) since they are connected together by contactor I5 when it is closed, Moreover, impedance legs 8 and 9 are connected by bridge terminal I511.

The control winding iii of the contactor I5 is c ntrol ed, by an electron discharge device or tube I! which is of the well-known tetrode type; as shown, it is provided with a cathode I8, a control grid I9 and a, screen grid 20 and also with an anode 2|. The anode-cathode circuit, which is the output circuit of this control device, controls the contactor winding I6; this winding, it will be observed, is connected in series with the anodecathode circuit. Connected across the terminals of the winding I6 is a capacitor 22 which prevents chattering of the contacts.

The anode-grid circuit, which is the input.

circuit of the device, is connected across the bridge terminals Ia and I4, I 4a, as shown. But connected in this circuit are a pair of resistors 23 and 24 of equal ohmic resistance, but having difierent thermal coeflicients of resistance. Resistor 23 has the higher thermal coeflicient and preferably will be made of the same material as heater 3, that is copper in the example illustrated, while resistor 24 preferably will be made of the same material as is heater 4, that is an alloy of copper and cadmium. And the ohmic resist ance of resistors 23 and 24 are relatively great as compared with the ohmic resistance of heaters 3 and 4. Thus for example while heaters 3 and 4 may have resistances of forty ohms the resistors 23 and 24 have resistances of five hundred ohms.

It will be observed that resistors 23 and 24 are thermally removed from the blanket body I; they are heated only by virtue of their energization, that is, their resistance to the passage 01' current.

The grid 20 and the cathode I8 of the discharge device I! are connected to point 25 between the resistors 23 and 24, and the resistors are connected in across the contactor I5 so that when the contactor is closed, the resistors are connected in parallel. Thus, if the contactor I5 is closed the parallel-connected and equal ohmic resistors 23 and 24 will have no effect on the operation of the tube II, but when the contactor is opened they do, as will be explained below.

In the operation of electron discharge devices of the character illustrated, it will be understood that the flow of current in the output anode circuit will be controlled by the potential of the grid of the input circuit. Assuming that the discharge device is of the character which will pass current, i. e., fire at one volt positive on the grid, if the grid is one volt positive or above, the discharge device will pass current through the anode circuit on each positive half cycle of the current, whereas if the grid voltage is more negative than one volt positive the device will not pass current. That is, if the voltage between bridge terminals I5a and 25 (25 in efiect being bridge terminal I 4, Ma) is more positive than one volt-the critical control grid voltage of the tubethe tube will fire, whereas if the voltage between these two bridge terminals becomes more negative than one volt, then the tube will not fire.

In this way, the control device I! will fire to pass current to energize the winding I 6 and thereby cause the contactor I5 to close to establish the bridge and energize heating elements 3 and 4, or will not pass current so as to deenergize the winding I6 and permit the contactor to open thereby to disable the bridge and substantially deenergize the heating elements 3 and 4.

The voltage of the grid circuit between bridge terminals I So and 25 depends upon the relative value of resistance of heaters 3 and 4 as t y are afiected by the temperature, or the blanket body. When the actual resistance of heater 3 is less than that of heater 4 due to a low blanket temperature, the voltage between terminal points I So and 25 is more positive than the critical control grid voltage and the tube will fire each positive half cycle so as to keep the contactor I5 closed. But if the actual value of resistance of heater 3 is greater than that of heater 4 then the voltage between the grid terminals I5a and 25 becomes more negative than the critical control grid voltage and the device I! will not fire. As a result the contactor I5 will open to shut down the blanket heat.

It will be understood in view of this that the temperature of the blanket body controls the character of the voltage generated by the bridgepositive or negativeto control the operation of the discharge device I! to control the heating means of the blanket through the contactor I6.

Preferably, the resistance 8, the resistances 9a and 9b, the transformer 21, the contactor I5, the tube IT, and the resistors 23 and 24, all will be located in a suitable casing 28 (Fig. 1). This casing will be connected to the blanket by a cord 29 which includes the two sets of wires connecting heaters 3 and 4 into the bridge, and will be connected with the supply source through a twin supply cord 30 having the usual twin terminal plug 3|.

In the operation of the system, when the blanket is plugged into the supply source and the blanket body I is cold and consequently the actual resistance of heater 3 is less than that of heater 4, the voltage between the bridge terminals I5a and 25, i. e., and I5a and I4, Man, will be sufficiently positive with respect to the critical grid voltage of tube I I to cause it to fire and thereby energize winding I 6 to hold contactor I5 closed. The heaters 3 and 4 supply heat energy to the blanket body I and as they continue to heat the body the body temperature, of course, increases. And as this temperature increases, the actual resistance of heater 3 rises, and it rises much faster than does the actual resistance of heater 4-assuming that the latter heater has a positive temperature coefiicient. Eventually when the blanket attains a predetermined high temperature, the value of actual resistance of heater 3 will have risen to such a high value that the voltage between bridge points I5a and 25 become more negative than the critical control grid voltage of the tube and the tube ceases to fire. This deenergizes the Winding I5 and contactor I5 opens. As soon as this happens the bridge comprising legs 3 and 4 and 8 and 9 is disabled, i. e., completely unbalanced, and it cannot then of itself restart the heating cycle.

This i accomplished by the resistors 23 and 24 which when the contactor I 5 opens are connected with impedance legs 8 and 9 to form a new Wheatstone bridge. As shown, when contactor I5 opens resistor 23 is connected in series with heater 4, whereas resistor 24 is connected in series with heater 3. And since the values of the ohmic resistance of resistors 23 and 24 are relatively high as compared with those of heaters 3 and 4 (23 and 24 having ohmic resistances of about five hundred in the example given above, whereas heaters 3 and 4 have an ohmic value of about forty), most of the voltage drop occurs across the resistors 23 and 24. That is, for all practical purposes heaters 3 and 4 are deenergized in the sense that they do not now apply any substantial degree of heat to the blanket body.

It will be remembered that resistor 23 has the higher temperature coeilicient of the two resistors 23 and 24. Therefore, since it is now connected with heater '4, the resistance characteristics of the two impedance legs of the new bridge as compared with those of the two heater legs of the first bridge are reversed, that is, the new bridge leg with heater 4 noW has the more positive temperature coefiicient, and in efiect may be said to be the same as when heater leg 3 of the first bridge is cold. Thus, when contactor l5 opens, resistors 23 and 24 are energized and while the blanket body and heaters 3 and 4 cool down, re-

sistors 23 and 24 heat up. The actual resistance of resistor 23 rises much more rapidly than does that of resistor 24 as the two resistances heat up, and as the blanket cools down the actual resistance of heater 3 decreases faster than does that of heater 4. As a result of these combined actionsthe voltage between bridge terminals [5a and 25 becomes less and less negative until it becomes sufficiently positive to cause the tube H to fire. This recloses contactor which cuts resistors 23 and 24 out of the control system, that is, it disables the second bridge and reestablishes the first bridge. Now, if the actual resistance of heater 3 is still too high, tube I! will not retire and contactor l5 will drop out to disable the first bridge and reestablish the second, and the tube I! will be refired by the action of the second bridge, as explained. This operation repeats, and the tube continues to cycle, the contactor l5 first closing and then opening, until the blanket body temperature has dropped sufiiciently low to lower the actual resistance of heater 3 to such a value that the first bridge when reestablished reclaims the control of the firing of the tube l1. Then the operation continues as before; the blanket heats up until contactor I5 is opened and then the second bridge causes recycling of the tube I! until the blanket temperature falls to a predetermined low degree, whereupon the first bridge when reestablished reclaims and holds the control until the temperature of the blanket again rises to the above-mentioned predetermined high value.

Should the blanket become folded up or should it otherwise attain a temperature too high, the control will function as described to cause the valve I! not to pass current and thereby in effect deenergize the heating means 3 and 4 until the whole blanket body cools down to a safe value. Also, in the foregoing fashion, the system will function responsively to rise and fall of the temperature of the blanket body responsively to room temperature variations to cycle the heating means between off and on positions to hold a substantially uniform temperature in the blanketand this in spite of variations in ambient temperature. Thus, if the ambient temperature falls, it will require more time to heat up the resistance 3 to the value at which it will cause the heating means to be shut down, and therefore, it will compensate and cause the heating means to hold the desired temperature. Conversely, if the ambient temperature rises less time will be required to heat the blanket up to the point at which the resistance 3 rises to the temperature value at which it causes the device I! to shut off the heat. In other words, the control will vary the proportion of time of each cycle that the heater is energized to the time that it is not, and thereby compensate for ambient temperature variations.

The blanket temperature setting may be adjusted by adjusting the value of the resistance 9b in the blanket impedance leg 9. Changing the value of resistance of 9b unbalances the bridge, which changes the firing point of the tube l1 and, therefore, the temperature of the blanket. Increasing this resistance, increases the voltage drop in leg 9 of the bridge which increases the voltage between points l5a and 25 (grid and cathode or I B and IS). The blanket leg 3 therefore has to become warmer to increase in resistance sufiiciently to obtain a high voltage drop across it in order to unbalance the bridge negatively anddeenergize the control relay l5. On decreasing this resistance in 9B, the action or operation is just the opposite; the voltage between points 15a and 25 is lowered so that the blanket, assuming it is cold on starting, does not have to heat up or increase in resistance as much to negatively unbalance the bridge.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric blanket comprising a flexible lanket body, a pair of resistance heating elements for heating said body, said elements having resistances which vary differentially with blanket temperature, and said two resistances constituting two impedance legs of a Wheatstone bridge, said bridge having two other impedance legs external of said blanket body, an electror'esponsive control element for controlling the energization of said heating elements connected to predetermined terminals of said bridge so a to respond to bridge voltage variations effected by the differential variations in the resistances of said heating elements responsively to blanket body temperature variations, said control element disabling said bridge when said temperature attains a predetermined high value, and auxiliary means operated responsively to the operation of said control element in disabling said first bridge for establishing a second bridge including said predetermined terminals so as to operate said control element periodically to reestablish said first bridge which functions when said blanket body temperature falls'to a predetermined low value to reclaim control of said control element and reapply heat to said blanket.

2. An electric blanket comprising a flexible blanket body, a pair of resistance heating elements for heating said body, said elements having resistances which Vary differentiall with blanket temperature, and said two resistances constituting two impedance legs of a Wheatstone bridge, said bridge having two other impedance legs external of said blanket body, the Voltage across two of the terminals of said bridge varying with variations in the respective resistances of said heating elements as said blanket body temperature varies, an electron discharge device provided with a cathode, an anode, and a controlling grid, a circuit including said grid and cathode connecting said two terminals, a control element for said heating elements connected in series with the anode-cathode circuit of said device so as to respond to the magnitude of the current thereof to disable said bridge to cut down the heat generated by said heating elements when the voltage across said two terminals at a predetermined high blanket body temperature becomes sufliciently negative with respect to the critical grid voltage of said discharge device, and means operated responsively to the disabling of said bridge for recycling said discharge device periodically to reestablish said bridge until said body temperature falls to a predetermined low value whereupon said bridge when reestablished reclaims control of said discharge device and reapplies heat to said blanket body.

3. An electric blanket comprising a flexible blanket body, a pair of resistance heating elements for heating said body, said elements having resistances which vary differentially with blanket temperature, and said two resistances constituting two impedance legs of a Wheatstone bridge, said bridge having two other impedance legs external of said blanket body, the voltage across two of the terminals of said bridge varying with variations in the respective resistances of said heating elements as said blanket body temperature varies, an electron discharge device provided with a cathode, an anode, and a controllin grid, a circuit including said grid and cathode connecting said two terminals, a contactor which when closed connects said heater elements into said bridge and when opened disables said bridge to cut down the heat supply to said blanket body, said contactor connected in series with the anode-cathode circuit of said device so as to respond to the magnitude of the current thereof to disable said bridge when at a predetermined high blanket body temperature the voltage across said two terminals becomes suiiiciently negative with respect to the critical control grid voltage of said device to cause it to cease firing, and a pair of additional resistors removed thermally from said blanket body connected in series with said heater elements respectively when said contactor opens so as to establish a second control bridge for said discharge device, said additional pair of resistors also having resistances which vary differentially to temperature change but in a sense reverse to that of said heating elements and in such degree as to cause said second bridge to cause said discharge device to fire to operate the contactor to close and thereby reestablish said first bridge, said contactor thereafter alternately opening and reclosing responsively to the operations of said first and second bridges until the blanket body temperature falls to a predetermined low value at which said first bridge retains control of the contactor until said blanket body again attains said predetermined high temperature.

4. An electric blanket comprising a flexible blanket body, a pair of heating resistance elements distributed through said blanket body for heating it, said resistance elements having resistances which vary diiferentially in response to temperature variations in said blanket body, an electrical supply circuit, an electroresponsive contactor which when closed connects said resistance elements in circuit with said supply circuit whereby they are energized to heat said body, a control system for said contactor including said heating elements when said contactor is closed and controlling said contactor to open when the relative resistance of said heating elements varies a predetermined degree in response to a predetermined high temperature condition in said blanket body, said control system also including a pair of resistors removed thermally from said blanket body and connected to be energized from said supply circuit only when said contactor opens, and said pair having resistances which vary difierentially in response to the temperature rise in said resistors when energized but in a sense reverse to that of said heating elements, whereby said control system then functions to cause said contactor to reclose when said resistances of said resistors vary by a predetermined degree responsively to temperature rise in said resistors, said contactor remaining closed in the event said relative resistances of said heating elements varies by a predetermined lesser amount than said predetermined degree,

5. An electric blanket comprising a flexible blanket body, a pair of heating elements in thermal relation with said body for applying heat thereto, an energizing circuit for said heating elements, a control contactor, a plurality of resistors forming two Wheatstone bridges for controlling said contactor, the first to open it and the second to reclose it, and the first established when said contactor is closed and the other established when it is opened, the first bridge including said pair of heating elements as two of its impedance legs and said legs having resistances which vary differentially with variations in the temperature of said blanket body and functioning to cause said bridge to open said contactor when said temperature attains a predetermined high value, and said second bridge including in two of its impedance legs a pair of resistors thermally removed from said blanket body and also havin resistances which vary differentially with temperature change but responding only to their own temperature rise when energized responsivel to the establishment of said second bridge, and said second bridge functioning to cause said contactor to reclose when the difference in the resistances of said resistors reaches a predetermined high value.

GEORGE C. CROWLEY. 

